Combustion and chemical processes generally generate gas streams containing contaminants that need cleanup before being exhausted to the atmosphere. Many industrial processes, power generating utilities, combustion processes, stationary and mobile sources such as engines, boilers, kilns and the like use solid fuels or low cost hydrocarbon fuels that contain sulfur, chlorine, nitrogen and metal compounds in hydrocarbons which result in exhaust gases that contain contaminants such as acid gases, particulate matter and heavy metals. To comply with stricter environmental rules mandated by legislation and a greater concern for the environment, combinations of scrubbing (wet or dry) and particulate capture devices such as electrostatic precipitators (ESP), wet ESP and bag houses are increasingly utilized for emissions control of acid gas and particulate matters.
Nitrogen oxides found in most combustion exhaust streams primarily are in the form of nitric oxide (NO), which is nearly insoluble in water and not very reactive. Nitric oxide is not removed to any significant extent by most wet or dry scrubber capture devices. To control nitrogen oxide emissions, therefore, the two major options are to lower nitrogen oxide formation at the source by modifying combustion or secondly treating nitrogen oxides in the exhaust gas stream using post combustion techniques.
Primary techniques used for reducing nitrogen oxide formation by modifying combustion are low nitrogen oxides burner (LNB), flue gas recirculation (FGR), staged combustion and over fire air (OFA). In most applications these technologies are not adequate for removing nitrogen oxides from combustion gas streams and post combustion techniques, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), become necessary to achieve the desired nitrogen oxide reduction levels.
Both SCR and SNCR processes realize good results but also have limitations. Ozone based oxidation technologies have recently gained success as an alternative post combustion technique, especially when an application is not suitable for SCR. Ozone based processes as described in U.S. Pat. Nos. 6,162,409; 5,206,002; and 7,303,735 provide multi-pollutant removal approaches.
Coal fired boilers with low nitrogen oxides burners and staged combustion often attain nitrogen oxides levels in the range of 0.25 to 0.4 lb/MMBTU (one million British Thermal Units) cost effectively whereas regulations require nitrogen oxides emissions to be in the range of 0.1 to 0.15 Ib/MMBTU.
The ozone based processes for oxidizing nitrogen oxides described in U.S. Pat. Nos. 5,206,002; 6,162,409; 6,649,132; and 7,303,735 are directed towards achieving high levels of nitrogen oxide removal (around 90%) and require the use of about 1.5 moles of ozone per mole of nitrogen oxide present in the exhaust gas stream. In the reaction time allowed in these methods, ozone reacts selectively with NOx forming higher oxides of nitrogen, especially the pentavalent form or higher which are very water soluble and readily removed by wet scrubbing. Configuring these processes to operate at lower levels of nitrogen oxide removal, however, causes both economic and process challenges.
The oxidation of NOx to N2O5 involves many reactions but for the sake of brevity, it can be simplified as follows:NO+O3→NO2+O2  (1)NO2+O3→NO3+O2  (2)NO2+NO3→N2O5  (3)
Reaction (1) is an order of magnitude faster than reaction (2). By the time reaction (2) starts to occur, most of the NO has already been oxidized to NO2. At levels of NOx removal of 90% and higher the actual molar ratio of ozone to NOx removed that is required is close to the stoichiometric ratio indicated above. At low to moderate levels of NOx removal, however, not only is significantly more ozone required than the ideal stoichiometric amount, but also the NOx that is emitted from the stack is essentially in the form of the brown colored and acrid smelling NO2.
U.S. Pat. No. 8,865,098 describes an ozone-based method for the partial removal of contaminants from a process gas stream that overcomes the economic and process limitations of the prior art ozone NOx oxidation processes.
In these methods the process gas stream containing contaminants is divided into at least two process gas streams. Ozone is injected into a selected one or more of the process gas streams for mixing of the ozone with the contaminants including nitrogen oxides. The nitrogen oxides in the selected process gas stream or streams are essentially fully oxidized by ozone. Then the ozone treated selected process gas stream or streams that are now substantially free of un-oxidized nitrogen oxides are recombined with the remaining process gas stream containing contaminants. The oxidized nitrogen oxides are removed by a capture device either from the selected process gas stream or streams prior to recombination with the remaining process gas stream containing contaminants, or after recombination of the gas streams.
Essentially 100% of the NOx in the selected gas stream or streams is thereby removed utilizing close to the stoichiometric molar ratio of ozone to NOx removed. No ozone is utilized oxidizing NO to NO2 in the un-treated stream and the NOx released from the stack remains mainly colorless NO. Both deficiencies in the earlier ozone NOx removal methods at low to moderate levels of NOx removal are hence addressed. In addition, since any small degree of ozone slip resulting from the high level of oxidation targeted in the one or more oxidized selected gas streams is immediately quenched by very reactive NO on recombination with the untreated gas stream.
The overall fraction of NOx removed is about equal to the fraction of the overall process stream contained in the at least one selected process gas streams to which ozone is added. U.S. Pat. No. 8,865,098 focuses on instances where a fixed proportion of the total NOx must be removed, e.g. 50%, or where a series of different fractions of NOx should be removed, e.g. 25%, 50% and 75% for example to meet current and future regulatory needs. Various embodiments are described that cost effectively achieve this, including: placing fixed partitions into existing exhaust gas ducts and or scrubbers and injecting ozone into one or more or the separated streams; injecting ozone into one of more of multiple ducts containing the exhaust gas stream from a combustion or chemical source of NOx contamination; and, injecting ozone into one or more separate zones in a spray drier, or other air pollution control equipment, so that a fixed proportion of the exhaust gas is treated with ozone. These embodiments work well and economically if emission regulations require that a fixed percentage (e.g. 75%) of NOx be removed from a source with constant or varying NOx levels, or that a specified NOx level be maintained in the stack from a source with a roughly constant NOx level (for example an inlet NOx level of about 400 mg/Nm3 be reduced to less than 100 mg/Nm3: in which case 75% of the exhaust gas is treated with ozone and then the oxidized products scrubbed out).
However, in the case of a source that generates a variable NOx level and where regulations require that a fixed level of NOx be maintained in the stack, then these partial NOx removal ozone oxidation solutions are not ideal. The required fraction of NOx removed, and hence the required proportion of the total process gas that must be treated varies continuously between the minimum fraction required to reduce the lowest NOx input level to the regulatory limit and the maximum fraction required to reduce the highest NOx inlet level to the regulatory level, not in a series of fixed increments, e.g., 25%, 50% and 75%.
U.S. Pat. No. 8,865,098 teaches a partial solution to this problem in the embodiment described in column 11, line 39 to column 12, line 9 as well as FIG. 8. This constitutes the nearest prior art to the current invention. In this embodiment a fan driven by a variable frequency drive is used to divert a variable proportion of the process gas stream to an ozone oxidation duct and scrubber that removes the oxidized NOx and other contaminants present, if any.
It is not feasible to implement this particular solution in many real world applications because of space limitations, the geometry of the process flow duct (more particularly if the plant has multiple ducts), or because of the impact of the wide range of different fractions of the process flow diverted through the fan on the pressure drop, the ozone mixing efficiency and mixing time and the residence time for the ozone oxidation in the ozone oxidation duct as well as scrubber efficiency.
The present inventors have discovered a flexible ozone-based oxidation system that can remove and capture a variable partial fraction of the nitrogen oxides contained in an exhaust gas. This variability can be in response to the source of nitrogen oxides contamination that continuously or stepwise varies with fuel or other operational parameters such as furnace load or changes in the required stack nitrogen oxides level.